1. Field of the Invention
This invention relates in general to servo operated control systems and more particularly, servo operated digital positioning control systems which are capable of being manufactured on a low cost basis and which utilize a phase shifting of a drive signal to obtain accurate servo positioning.
2. Brief Description of the Prior Art
High accuracy positioning systems have been used for many years in a variety of military and aerospace applications to position moveable control members. Such applications include the positioning of antennas and optical sensors. A key element in any positioning system is the position transducer used to sense the position of the moveable control element since the overall performance of the positioning system is a function of the accuracy, repeatability and linearity of the position transducer. Historically, many types of transducers have been employed to measure position, such as potentiometers, resolvers, differential transformers, magnetic encoders, optical encoders and the like.
Optical shaft encoders have found widespread use as position transducers in high accuracy positioning systems. In particular, absolute position optical encoders are employed to obtain the high resolution necessary for accurate positioning. These types of encoders include a light source, such as a light emitting diode, a group of photo-sensors, such as photo-diodes, and a coded disc positioned between the light source and the photo-sensors. If the control element to be positioned is the shaft of a motor, the coded disc is rotatably mounted to the motor shaft. The coded disc includes patterns of clear and opaque segments which vary in size and location according to the resolution required by the application. The patterns are arranged so that light passes from the light source through the clear segments of the disc and illuminates the photo-sensors in a prescribed manner whereby the photo-sensor output signals form a binary representation of the absolute position of the motor shaft.
The coded disc is usually formed of glass and the patterns are photographically imprinted on the disc to create the clear and opaque areas. As the disc rotates with the motor shaft, the photo-sensors detect the absence or presence of light as a function of both the disc patterns and the position of the disc. The disc patterns are configured so that the photo-sensor output signals may be combined to form a digital number using a code, such as binary coded decimal or gray binary code. The resolution of the encoder determines the number of bits required in the digital number.
The size of the disc, the required accuracy of the pattern, and the required number of photo-sensors all increase as the resolution requirement of the encoder increases. For example, to resolve one revolution of the motor shaft into a thousand parts requires a coded disc of approximately two and one-half inches in diameter and ten photo-sensors. Higher resolutions require larger discs, extremely accurate patterns, and a proportionately greater number of photo-sensors. The mechanical alignment of the disc, light source and photo-sensors also becomes a critical factor in higher resolution encoders. Because of the critical alignment requirements, as well as the large number of components employed in their construction, absolute position optical encoders are expensive to produce. Accordingly, the use of these encoders has been limited primarily to military and aerospace applications, as opposed to commercial and industrial applications.
Another type of optical encoder which has found widespread use in positioning systems is the incremental position encoder. Incremental position encoders are less expensive to produce than absolute position encoders, but suffer from limited resolution. This type of encoder also employs a light source and a group of photo-sensors. Mounted in a parallel, spaced-apart relationship between the light source and the photo-sensors is a moveable control element such as a motor shaft. The disc and the reticle are each provided with a pattern of clear apertures and opaque areas. As the moveable disc rotates with the motor shaft, light paths from the light source to the photo-sensors are created by the juxtaposition of the apertures in the disc and reticle. The light paths are detected by the photo-sensors which are positioned with respect to the reticle to create two output signals in response to the rotation of the moving disc. These two signals are typically in the form of sine or square waves displaced in phase ninety degrees with respect to each other. Each of the sine or square waves represents the incremental rotation of the moveable disc by a distance equal to the spacings between apertures.
Counting the number of sine or square waves that occur as the motor shaft rotates provides a determination of the position of the shaft. Prior art techniques have also been developed for counting the number of zero crossings which occur in the encoder output signals. These techniques permit resolving the position of the motor shaft into four parts for each of the apertures in the disc pattern. The resolution thus obtainable in the use of incremental shaft encoders has been limited to four times the number of apertures which can be formed by the pattern on the discs. Incremental encoders are typically limited to disc patterns having a maximum of one thousand apertures per inch. Increasing the number of apertures beyond this number is impractical because of the extremely accurate tolerances required in the manufacture and assembly of such encoders. Resolving each of the apertures into four parts as described above results in a resolution of one part in ten thousand using incremental position encoders. On the other hand, high accuracy positioning systems may require resolutions of one part in one million. Therefore, incremental encoders have been limited to use in positioning systems which do not require high positioning accuracy.
In my co-pending patent application Ser. No. 275,926, filed June 22, 1981, entitled "Digital Positioning Systems Having Accuracy", there is provided a digital servo control system which utilizes incremental position optical encoders as position transducers for obtaining high position accuracy. This system generates a highly accurate digital position signal in response to output signals from an incremental optical encoder. While this system is highly effective in achieving accurate positioning, it is possible to utilize a lower cost digital positioning servo system for any applications, as for example, in printer control drive systems and the like. Further, it is possible to modify an existing servo control system in order to substantially improve the accuracy thereof.